V-ribbed belt and method for fabricating the same

ABSTRACT

A V-ribbed belt B has bottom and back faces each provided with a plurality of ribs  3, 4  of approximately trapezoidal section which extend in parallel with each other along the belt length. In order to suppressing a discrepancy between respective speed ratios of revolution transmitted from one pulley to the other in normal and reversed positions of the belt B, the belt B is adapted to have a difference of 2 mm or less between a bottom-face-side belt length when the bottom-face-side ribs  3  of the belt B are faced on an inner peripheral side thereof and a back-face-side belt length when the back-face-side ribs  4  of the belt B are faced on the inner peripheral side thereof. The center of a cord  2  embedded inside of the belt B is thereby positioned substantially in the middle of the belt thickness so that the cord  2  has equal distances from both the bottom face and back face of the belt B even if the belt B is in either position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a V-ribbed belt having bottom and back faceseach provided with the same number of ribs and relates to a method forfabricating the V-ribbed belt.

2. Description of the Prior Art

As an example of V-ribbed belts of this type, there is conventionallyknown one in which the bottom and back faces of the belt each have thesame number of ribs of approximately trapezoidal section formed incorresponding positional relation between the bottom and back faces, forexample, as disclosed in U.S. Pat. Nos. 5,273,496, 5,334,107 and5,507,699.

More specifically, if the top of each rib has an arcuate section, rightand left side faces of each rib will act as contact portions in contactunder pressure with side faces of a pulley groove of a V-ribbed pulleyto contribute to power transmission. In this case, however, the top ofeach rib not in contact with any surface of the pulley groove forms anunnecessary portion not required for power transmission, and theunnecessary portion induces a problem that it causes cracks due tobending of the belt. Therefore, in the above conventional example, theunnecessary portion at the rib top is eliminated by forming the rib intoan approximate trapezoidal section.

Meanwhile, this double-sided V-ribbed belt may be used in a normalposition in which the bottom face of the belt is used as an innerperipheral side thereof, and may alternatively be used in a reversedmanner that the belt is turned inside out such that the original backface thereof is used as an inner peripheral side.

In such a reversed position of the belt, if the cord is not positionedat the substantially middle portion of the belt along the beltthickness, the relative position of the cord to the pulley when the beltis wound around the pulley will be changed between the normal position(see FIG. 5a) and the reversed position (see FIG. 5b). This increases adifference between an effective pulley diameter when thebottom-face-side ribs of the V-ribbed belt are faced on the innerperipheral side thereof and an effective pulley diameter when theback-face-side ribs thereof are faced on the inner peripheral sidethereof, resulting in an increased departure of a speed ratio ofrevolution transmitted between the pulleys of the effective pulleydiameters from that of revolution transmitted between pulleys of designdiameters. In addition, the belt length will also be changed between thebottom and back faces. This needs for a large movement allowance of thepulley movable for tensioning the belt and therefore makes it difficultto provide a compact design of a belt power transmission device.

Particularly for a belt in which the bottom-face-side and back-face-sideribs are arranged at equal pitches so as to be used arbitrarily ineither of its normal and reversed positions, an influence derived fromthe departure from the speed ratio as designed becomes a seriousproblem. It causes, for example, lack of capacity of an engine auxiliary(a decreased amount of power generation or a decreased hydraulicpressure), deterioration in shaft life due to abnormal increases inrotational speed, and so on.

Therefore, the present invention has its object of suppressing adiscrepancy between respective speed ratios of revolution transmittedfrom one pulley to the other in the normal and reversed positions of aV-ribbed belt having ribs of approximately trapezoidal section asdescribed above on both the bottom and back faces by appropriatelypositioning a cord of the belt.

SUMMARY OF THE INVENTION

To attain the above object, the present invention provides forpositioning the center of the cord substantially in the middle of thebelt thickness.

More specifically, the present invention is directed to a V-ribbed beltin which a cord is embedded generally in a helical arrangement to form arow along the belt width and a plurality of ribs of approximatelytrapezoidal section are formed on each of bottom and back faces of thebelt and in corresponding positional relation between the bottom andback faces to extend in parallel with each other along the belt length.

In the belt, the ribs in both the bottom and back faces are made ofrubber of identical characteristic and formed in approximately identicalsize and shape, and the cord is disposed substantially in the middle ofthe belt thickness.

With this arrangement, since the cord is disposed substantially in themiddle of the belt thickness, the relative position of the cord to apulley when the belt is wound around the pulley in each of the normaland reversed positions is identical. This suppresses a discrepancybetween the speed ratios of one pulley to the other in both thepositions of the belt.

A difference between a belt length when the ribs in the bottom face ofthe belt are faced on an inner peripheral side thereof and a belt lengthwhen the ribs in the back face thereof are faced on the inner peripheralside thereof is preferably 2 mm or less. With this arrangement, therecan be obtained a desirable range of positions of the cord to bedisposed substantially in the middle of the belt thickness. In otherwords, if the difference between both the belt lengths is over 2 mm, aneffect of suppressing a discrepancy in speed ratio between the normaland reversed positions of the belt is insufficient. Therefore, thedifference between both the belt lengths is set at 2 mm or less.

A difference between a distance from the center of the cord to a top endof each of the ribs in the bottom face and a distance from the center ofthe cord to a top end of each of the corresponding ribs in the back facemay be 0.3 mm or less on the average over the entire circumference ofthe belt. Also in this case, there can be obtained a desirable range ofpositions of the cord to be disposed substantially in the middle of thebelt thickness. In other words, if the difference between both thedistances is over 0.3 mm on the average over the entire circumference ofthe belt, the effect of suppressing the discrepancy in speed ratiobetween the normal and reversed positions of the belt is insufficient.Therefore, the difference between both the distances is set at 0.3 mm orless.

A top of the rib may include a planar top surface along the belt widthand a pair of right and left corners with one end continuing to a leftor right end of the top surface and the other end continuing to a sideface of the rib, both the corners may be formed in respective arcuatesections having different centers of curvature positioned bilaterallywith respect to the centerline of the rib, and when the height of therib is hr and the curvature radius of the corner of the rib is R, thecurvature radius R may be set to meet the following formula: 0.17hr≦R<0.5 hr.

Alternatively, a top of the rib may include a planar top surface alongthe belt width and a pair of right and left corners with one endcontinuing to a left or right end of the top surface and the other endcontinuing to a side face of the rib, both the corners may be formed inrespective arcuate sections having different centers of curvaturepositioned bilaterally with respect to the centerline of the rib, andwhen the pitch of the ribs is p and the curvature radius of the cornerof the rib is R, the curvature radius R may be set to meet the following

formula: 0.14 p≦R<0.35 p.

With these arrangements, only the right and left corners of each rib ofthe belt located on both sides of the planar top surface of the rib areformed in respective arcuate sections having different centers ofcurvature positioned bilaterally with respect to the centerline of therib, and the curvature radius R of the corner falls into the range of0.17 hr≦R<0.5 hr or the range of 0.14 p≦R<0.35 p. Accordingly, when thebelt is in a bent position, it can be effectively prevented that stressis concentrated to the corners of each rib located in the outerperiphery of the belt. This improves crack-proof performance of the beltin its bent position and keeps it constant, thereby extending the beltlife. In addition, large areas of the rib side faces can be kept intocontact with the pulley groove when the belt is fitted into the pulleygroove. This ensures an improved power transmission performance of thebelt.

If the curvature radius R of the rib corner is less than 0.17 hr or lessthan 0.14 p, the crack-proof performance of the belt will beinsufficient. On the other hand, if the curvature radius R of the ribcorner is not less than 0.5 hr or not less than 0.35 p, not only thecrack-proof performance of the belt will be insufficient but also therate of slip thereof will be increased because the length of the contactportion of the rib in contact under pressure with the pulley groovesurface to contribute to power transmission is decreased. Therefore,when the curvature radius R of the rib corner is set to fall into therange of 0.17 hr≦R<0.5 hr or the range of 0.14 p≦R<0.35 p, the belt canimprove its crack-proof performance at the rib corner located at the topof the rib while improving its power transmission performance byincreasing the area of the contact portion of the rib contributing topower transmission.

Furthermore, the present invention is directed to a method forfabricating a V-ribbed belt in which a cord is embedded generally in ahelical arrangement to form a row along the belt width and a pluralityof ribs of approximately trapezoidal section are formed on each ofbottom and back faces of the belt and in corresponding positionalrelation between the bottom and back faces to extend in parallel witheach other along the belt length. This method includes the steps ofgrinding a bottom face of a flat belt including a cord embedded thereingenerally in a helical arrangement to form a row along the belt widthuntil a thickness d1 of the belt after ground satisfies the followingformula:

d 1=hr+δc+C/2+H/2+(L 1−L 2)/(2π)  {circle around (1)}

where hr is a set height of the rib, δc is a set thickness from an endof the cord adjacent the rib to a bottom of the rib, C is the diameterof the cord, H is a set thickness of the V-ribbed belt, L1 is abottom-face-side inner length of the belt when the bottom face of thebelt is faced on an inner peripheral side thereof, and L2 is aback-face-side inner length of the belt when the back face of the beltis faced on the inner peripheral side thereof, and grinding a back faceof the ground flat belt until a thickness d2 of the belt after groundsatisfies the following formula:

d 2=(hr+δc+C/2)×2  {circle around (2)}

According to this method, when the V-ribbed belt is fabricated bygrinding both faces of the flat belt to form ribs therein, both thefaces of the flat belt can be ground such that the cord is positionedsubstantially in the middle of the thickness of the V-ribbed belt evenif the position of the cord embedded in the flat belt is unknown.Accordingly, a V-ribbed belt having a cord positioned substantially inthe middle of the belt thickness can be readily obtained.

In the above case, in obtaining the bottom-face-side inner length L1 andthe back-face-side inner length L2 of the belt, the bottom-face-sideinner length L1 of the belt is preferably obtained with the flat beltyet to be ground wound at the bottom face thereof between measuringpulleys formed of a pair of flat pulleys having equal diametersaccording to the following formula:

L 1=(2×CD 1 +Kπ)/α  {circle around (3)}

where CD1 is the center distance between the measuring pulleys, K is theouter diameter of the measuring pulley, and α is the coefficient ofextension of the flat belt under a load of the measuring pulley, andthen the back-face-side inner length L2 of the belt is preferablyobtained with the flat belt wound at the back face thereof between themeasuring pulleys according to the following formula:

L 2=(2×CD 2 +Kπ)/α  {circle around (4)}

where CD2 is the center distance between the measuring pulleys.

Thus, by measuring the center distances CD1 and CD2 between themeasuring pulleys, the bottom-face-side and back-face-side inner lengthsL1 and L2 of the belt can be readily obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a V-ribbed belt according to anembodiment of the present invention.

FIG. 2 is an enlarged cross-sectional view of a rib.

FIG. 3 is an illustrative diagram showing a yet-to-be-machined flat beltin a state wound between measuring pulleys to measure the centerdistance.

FIG. 4 shows cross-sectional views for illustrating the thicknesses ofthe flat belt after ground.

FIG. 5 shows cross-sectional views of a V-ribbed belt in which thedistance between a cord center and a bottom-face-side rib top isdifferent from the distance between the cord center and a back-face-siderib top, the V-ribbed belt being wound around a pulley in normal andreversed positions.

FIG. 6 is a graph showing results of an examination for finding arelationship of the difference in belt length with the differencebetween the distance from the cord center to a bottom face of the beltand the distance from the cord center to a back face of the belt.

FIG. 7 is a graph showing results of an examination for finding arelationship of the rate of departure of a speed ratio with thedifference between the distance from the cord center to the bottom faceof the belt and the distance from the cord center to the back face ofthe belt.

FIG. 8 is an illustrative diagram showing an auxiliary driving devicefor examining a relationship of the change in the amount of powergenerated by an alternator with the difference between the distance fromthe cord center to the bottom face of the belt and the distance from thecord center to the back face of the belt.

FIG. 9 is a graph showing results of examinations for finding respectiverelationships of the number of cycles taken for the belt to produce acrack and the rate of slip of the belt with respect to the curvatureradius of a corner of the rib.

FIG. 10 is a graph showing results of another examination for findingthe relationship of the number of cycles taken for the belt to produce acrack with respect to the curvature radius of the corner of the rib.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a V-ribbed belt B according to an embodiment of the presentinvention. This belt B is formed of a double-sided V-ribbed belt with athickness of H. In the figure, a reference numeral 1 denotes an endlessbelt base made of rubber. In the belt base 1, a cord 2 with a diameterof C is embedded generally in a helical arrangement to form a row alongthe belt width (in a lateral direction of FIG. 1).

On the bottom face of the belt base 1, a plurality of, for example,three bottom-face-side ribs 3, 3, . . . of approximately trapezoidalsection and with a height of hr1 are formed to extend in parallel witheach other along the belt length and at regular pitches p along the beltwidth. On the other hand, on the back face of the belt base 1,back-face-side ribs 4, 4, . . . , which is of approximately trapezoidalsection, with a height of hr2 and identical in number with thebottom-face-side ribs 3, 3, . . . , are formed at the same pitches palong the belt width as in the bottom-face-side ribs 3, 3, . . . tocorrespond to them in a vertical relationship.

The bottom-face-side ribs 3, 3, . . . and the back-face-side ribs 4, 4,. . . are made of rubber having identical characteristics (for example,abrasion resistance and heat resistance) and formed substantially in thesame size. The cord 2 is disposed substantially in the middle of thebelt thickness.

More specifically, a difference between a belt length when thebottom-face-side ribs 3, 3, . . . are face on an inner peripheral sideof the belt (see FIG. 5a) and a belt length when the back-face-side ribs4, 4, . . . are faced on the inner peripheral side of the belt (see FIG.5b) is set at 2 mm or less. Further, a difference |hc1−hc2| between adistance hc1 from the center of the cord 2 to the top end of each of thebottom-face-side ribs 3, 3, . . . and a distance hc2 from the center ofthe cord 2 to the top end of each of the corresponding back-face-sideribs 4, 4, . . . is set at 0.3 mm or less on the average over the entirecircumference of the belt.

Furthermore, as illustrated in detail and enlarged dimension in FIG. 2,the top of each of the bottom-face-side ribs 3, 3, . . . and theback-face-side ribs 4, 4, . . . includes a planar top surface 6 alongthe belt width (the direction of arrangement of the cord 2) and a pairof right and left corners 8, 8 with one end continuing to a left orright end of the top surface 6 and the other end continuing to a ribside face 7. Both the corners 8, 8 are formed in respective arcuatesections having two different centers 0, 0 of curvature positionedbilaterally with respect to the vertical centerline L of the ribs 3, 4.

In addition, both the corners 8, 8 have equal curvature radii R, R. And,when the height of the rib 3, 4 is hr (=hr1 or hr2), the curvatureradius R is set to fall into the range of 0.17 hr≦R<0.5 hr or the rangeof 0.14 p≦R<0.35 p.

Next, a method for fabricating the V-ribbed belt B will be described. Asshown in FIG. 3, a flat belt B′ including the cord 2 embedded thereingenerally in a helical arrangement to form a row along the belt width isprepared, and the flat belt B′ is wound between a pair of measuringpulleys 11, 12 each formed of a flat pulley with an outer diameter of Kso as to provide contact at the bottom face of the belt B′ with themeasuring pulleys 11, 12. In this state, a measuring load W is appliedto one of the measuring pulleys, for example, the right-hand one 11 inFIG. 3, in a direction away from the other measuring pulley 12, and acenter distance CD1 between both the measuring pulleys 11, 12 ismeasured. Based on the center distance CD1, a bottom-face-side innerlength L1 of the belt when the bottom face of the V-ribbed belt B isfaced on the inner peripheral side thereof is found by L1=(2×CD1+K π)/α(see the above-described formula {circle around (3)}), wherein α is thecoefficient of extension of the flat belt B′ under the measuring load W.

Subsequently, the flat belt B′ is turned inside out to change the backface thereof to the inner peripheral side (the bottom face to the outerperipheral side), and in this reversed state the flat belt B′ is woundbetween the same measuring pulleys 11, 12 so as to provide contact atthe back face thereof with the measuring pulleys 11, 12. And, in thesame manner as described above, a center distance CD2 between themeasuring pulleys 11, 12 is measured with a measuring load W applied tothe measuring pulley 11. Then, based on the measured center distanceCD2, the back-face-side inner length L2 of the belt when the back faceof the V-ribbed belt B is faced on the inner peripheral side is found byL2=(2×CD2+K π)/α (see the above-described formula {circle around (4)}).

Thereafter, as shown in FIG. 4a, the bottom face of the flat belt B′ noworiented on the outer peripheral side is ground by a grinding stone 13as it is put into the reversed state. The grinding work is performeduntil a thickness d1 of the belt after ground reacheshr+δc+C/2+H/2+(L1−L2)/(2π) (see the above-described formula {circlearound (1)}), namely, if the center distances CD1 and CD2 are used,hr+δc+C/2+H/2+(CD1−CD2)/(π α), wherein δc is a set thickness from an endof the cord 2 adjacent the rib 3, 4 to the bottom of the rib 3, 4.

Subsequently, as shown in FIG. 4b, the ground flat belt B′ is turnedfrom the reversed state back to the original state, and the back facethereof oriented on the outer peripheral side is ground by the grindingstone 13 in the same manner. The grinding work is performed until athickness d2 of the belt after ground reaches (hr+δc+C/2)×2 (see theabove-described formula {circle around (2)}).

As described above, in the present embodiment, the difference betweenthe bottom-face-side belt length when the bottom-face-side ribs 3, 3, .. . of the V-ribbed belt B are faced on the inner peripheral side of thebelt and the back-face-side belt length when the back-face-side ribs 4,4, . . . thereof are faced on the inner peripheral side thereof is setat 2 mm or less. Further, the difference |hc1−hc2| between the distancehc1 from the center of the cord 2 to the top end of each of thebottom-face-side ribs 3, 3, . . . and the distance hc2 from the centerof the cord 2 to the top end of each of the corresponding back-face-sideribs 4, 4, . . . is set at 0.3 mm or less on the average over the entirecircumference of the belt B. For these reasons, the center of the cord 2can be positioned substantially in the middle of the belt thickness.Accordingly, the position of the cord 2 in the normal position and thatin the reversed position of the belt are identical in the state that thebelt B is wound between the V-ribbed pulleys. As a result, a differencebetween an effective pulley diameter DC1 when the bottom-face-side ribs3 of the V-ribbed belt B are faced on the inner peripheral side thereofand an effective pulley diameter DC2 when the back-face-side ribs 4 ofthe belt are faced on the inner peripheral side thereof is small. Thissuppresses a departure of the speed ratio between the pulleys of theeffective diameters in each of the positions of the belt from the speedratio between pulleys of design diameter D.

Furthermore, only the right and left corners 8, 8 located on both sidesof the planar top surface 6 of each of the ribs 3, 4 of the belt B areformed in respective arcuate sections having different centers 0, 0 ofcurvature positioned bilaterally with respect to the centerline of theribs 3, 4, and the curvature radius R of each of the corners 8, 8 is setwithin the range of 0.17 hr≦R<0.5 hr or the range of 0.14 p≦R<0.35 p.Accordingly, when the belt B is in a bent position, it can beeffectively prevented that stress is concentrated to the corners 8, 8 ofeach rib 3 or 4 located in the outer periphery of the belt. Thisimproves crack-proof performance of the belt B in its bent position andkeeps it constant, thereby extending the lifetime of the belt B. Inaddition, large areas of the rib side faces 7, 7 can be kept intocontact with the pulley groove when the belt is fitted into the pulleygroove. This ensures an improved power transmission performance of thebelt B.

Moreover, when the V-ribbed belt B is fabricated by grinding the flatbelt B′, the bottom face of the flat belt B′ is first ground until thethickness d1 of the belt after ground reaches hr+δc+C/2+H/2+(L1−L2)/(2π)and the back face of the ground flat belt B′ is then ground until thethickness d2 of the belt after ground reaches (hr+c+C/2)×2. Therefore,both the bottom and back faces of the flat belt B′ can be ground suchthat the cord 2 is positioned substantially in the middle of thethickness of the V-ribbed belt B even if the position of the cord 2embedded in a flat belt B′ is unknown. Accordingly, a V-ribbed belt Bhaving a cord 2 positioned substantially in the middle of the beltthickness can be readily obtained.

In this case, the bottom-face-side inner length L1 of the belt isobtained with the flat belt B′ yet to be ground wound between the pairof measuring pulleys 11, 12 having equal diameters so as to providecontact at the bottom face thereof with the measuring pulleys 11, 12,and the back-face-side inner length L2 of the belt is obtained with theflat belt B′ wound between the measuring pulleys 11, 12 so as to providecontact at the back face thereof with the measuring pulleys 11, 12.Accordingly, these inner lengths L1 and L2 of the belt can be readilyobtained.

Next, examples practically made will be described.

(Examination 1)

For the V-ribbed belt having the structure of the above embodiment,various belt examples was fabricated in such a manner that the distancehc1 from the center of the cord to the top end of the bottom-face-siderib and the distance hc2 from the center of the cord to the top end ofthe back-face-side rib were changed respectively. Each of the distanceshc1 and hc2 is an average over the entire circumference of the belt.Then, on an X-Y two-dimensional coordinate basis, V-ribbed pulleys withan outer diameter of 120 mm were disposed at positions represented bycoordinates (0, 0) and (0, 300), respectively, a V-ribbed pulley with anouter diameter of 55 mm was disposed at a position represented by acoordinate (200, 150), and a V-ribbed pulley with an outer diameter of75 mm was disposed at a position represented by a coordinate (50, 150)(wherein the unit of each coordinate is mm). Each of the belt examplesdescribed above was wound around these pulleys in each of the normal andreversed positions, and a difference between lengths (at the cordposition) of the belt example wound respectively in both the positionswas measured. FIGS. 5a and 5 b show the states of the belt wound arounda pulley in normal and reversed positions, respectively. It is to benoted that like parts as in FIG. 1 will be described using likereference numerals. The examination results are shown in Table 1 andFIG. 6.

TABLE 1 Belt Belt length length in Difference Difference in normalreversed in belt hc1 hc2 in hc position position length 2.95 3.95 1.001279.22 1272.93 6.28 3.05 3.85 0.80 1278.59 1273.56 5.03 3.15 3.75 0.601277.96 1274.19 3.77 3.25 3.65 0.40 1277.33 1274.82 2.51 3.35 3.55 0.201276.70 1275.45 1.26 3.45 3.45 0.00 1276.08 1276.08 0.00 (unit: mm)

As can be seen from Table 1 and FIG. 6, the difference in belt lengthbetween the normal and reversed positions is larger as the difference|hc1−hc2| between the distance hc1 from the center of the cord to thetop end of the bottom-face-side rib and the distance hc2 from the centerof the cord to the top end of the back-face-side rib is increased. Itcan be also seen therefrom that the corresponding difference in beltlength is approximately 2 mm when the difference |hc1−hc2| is 0.3 mm.

(Examination 2)

Like Examination 1, various belt examples were prepared in a manner thatthe distance hc1 from the center of the cord to the top end of thebottom-face-side rib and the distance hc2 from the center of the cord tothe top end of the back-face-side rib were respectively changed. Each ofthese examples was wound between drive and driven pulleys having equalouter diameters of 100 mm in each of the normal and reversed positions,and the rate of departure of the speed ratio in the reversed positionfrom that in the normal position was determined. The same examinationwas also executed in the condition that the drive and driven pulleyshaving equal diameters of 50 mm are used. The results of theseexaminations are shown in Table 2 and FIG. 7.

TABLE 2 Normal position Reversed position Rate of Eff. OD Eff. OD Eff.OD Eff. OD Departure OD of OD of of of Speed of of Speed of speed drivedriven Diff. drive driven ratio drive driven ratio ratio (%) pulleypulley hc1 hc2 in hc pulley pulley a pulley pulley b (b/a) × 100 100 1002.95 3.95 1.00 101.7 103.7 0.981 103.7 101.7 1.020 104.0 3.00 3.90 0.90101.8 103.6 0.983 103.6 101.8 1.018 103.6 3.05 3.85 0.80 101.9 103.50.985 103.5 101.9 1.016 103.2 3.10 3.80 0.70 102.0 103.4 0.986 103.4102.0 1.014 102.8 3.15 3.75 0.60 102.1 103.3 0.988 103.3 102.1 1.012102.4 3.20 3.70 0.50 102.2 103.2 0.990 103.2 102.2 1.010 102.0 3.25 3.650.40 102.3 103.1 0.992 103.1 102.3 1.008 101.6 3.30 3.60 0.30 102.4103.0 0.994 103.0 102.4 1.006 101.2 3.35 3.55 0.20 102.5 102.9 0.996102.9 102.5 1.004 100.8 3.40 3.50 0.10 102.6 102.8 0.998 102.8 102.61.002 100.4 3.45 3.45 0.00 102.7 102.7 1.000 102.7 102.7 1.000 100.0 5050 2.95 3.95 1.00 51.7 53.7 0.963 53.7 51.7 1.039 107.9 3.00 3.90 0.9051.8 53.6 0.966 53.6 51.8 1.035 107.1 3.05 3.85 0.80 51.9 53.5 0.97053.5 51.9 1.031 106.3 3.10 3.80 0.70 52.0 53.4 0.974 53.4 52.0 1.027105.5 3.15 3.75 0.60 52.1 53.3 0.977 53.3 52.1 1.023 104.7 3.20 3.700.50 52.2 53.2 0.981 53.2 52.2 1.019 103.9 3.25 3.65 0.40 52.3 53.10.985 53.1 52.3 1.015 103.1 3.30 3.60 0.30 52.4 53.0 0.989 53.0 52.41.011 102.3 3.35 3.55 0.20 52.5 52.9 0.992 52.9 52.5 1.008 101.5 3.403.50 0.10 52.6 52.8 0.996 52.8 52.6 1.004 100.8 3.45 3.45 0.00 52.7 52.71.000 52.7 52.7 1.000 100.0 (unit: mm)(

As can be seen from Table 2 and FIG. 7, the discrepancy in speed ratiobetween the normal and reversed positions is larger as the difference|hc1−hc2| between the distance hc1 from the center of the cord to thetop end of the bottom-face-side rib and the distance hc2 from the centerof the cord to the top end of the back-face-side rib is increased.

(Examination 3)

Like Examinations 1 and 2, various belt examples were prepared in amanner that the distance hc1 from the center of the cord to the top endof the bottom-face-side rib and the distance hc2 from the center of thecord to the top end of the back-face-side rib were respectively changed.Each of these examples was wound around pulleys of an auxiliary drivingdevice for an engine in each of the normal position (a normally facingbelt drive) and reversed position (a reverse facing belt drive), and achange in speed ratio between both the positions and an associatedchange in amount of power generated by an alternator (not shown) as anauxiliary machine were examined. The results of this examination areshown in Table 3. It is to be noted that as shown in FIG. 8, theauxiliary driving device is disposed so that four idle pulleys 22through 25 are arranged between a drive pulley 20 with an outer diameterof 150 mm and a driven pulley 21 with an outer diameter of 55 mm with,for example, one pulley 22 forming tight side spans and the otherpulleys forming slack side spans. The drive pulley 20 is drivinglyconnected to a crank shaft of the unshown engine to serve as a crankpulley, while the driven pulley 21 is drivingly connected to thealternator to serve as an alternator pulley. The belt B is wound in “anormally bent state” around the drive pulley 20 and the idle pulleys 22through 24, while it is wound in “a reverse bent state” around theremaining driven pulley 21 and idle pulley 25.

TABLE 3 Normally facing belt drive Reverse facing belt drive Diff. inAmount Amount amount Eff. Eff. of Eff. Eff. of of drive driven gener-drive driven gener- gener- OD of OD of pulley pulley Eff. ated pulleypulley Eff. ated ated drive driven Diff. diam. diam. speed power diam.diam. speed power power pulley pulley hc1 hc2 in hc (mm) (mm) ratio (A)(mm) (mm) ratio (A) (A) 150 55 2.85 4.05 −1.20 151.6 57.8 2.62 42.0152.8 56.6 2.70 46.5 4.5 2.93 3.98 −1.05 151.7 57.7 2.63 43.0 152.7 56.72.69 46.0 3.0 3.00 3.90 −0.90 151.8 57.7 2.63 43.5 152.7 56.8 2.69 46.02.5 3.08 3.83 −0.75 151.8 57.6 2.64 44.5 152.6 56.8 2.68 45.5 1.0 3.153.75 −0.60 151.9 57.5 2.64 44.5 152.5 56.9 2.68 45.5 1.0 3.23 3.68 −0.45152.0 57.4 2.65 44.5 152.4 57.0 2.68 45.5 1.0 3.30 3.60 −0.30 152.1 57.42.65 45.0 152.4 57.1 2.67 45.5 0.5 3.38 3.53 −0.15 152.1 57.3 2.66 45.0152.3 57.1 2.67 45.0 0.0 3.45 3.45 0.00 152.2 57.2 2.66 45.0 152.2 57.22.66 45.0 0.0

As can be seen from Table 3, when the difference |hc1−hc2| between thedistance hc1 from the center of the cord to the top end of thebottom-face-side rib and the distance hc2 from the center of the cord tothe top end of the back-face-side rib is 0.3 mm or less, the amount ofpower generated by the alternator can be held at 45 amperes or more withstability even if the belt B is run in the normal position (the normallyfacing belt drive) or in the reverse position (the reverse facing beltdrive), and therefore a difference of 0.3 mm or less between thedistances hc1 and hc2 provides a suitable range for positioning the cordsubstantially in the middle of the belt thickness.

(Examination 4)

As a practical example of the present invention, a sample V-ribbed beltwas prepared in which the rib top thereof was formed of a planar topface and a pair of right and left corners located on both sides of thetop face as described in the above embodiment. In this belt, the numberof bottom-face-side ribs 3 was three, the number of back-face-side ribs4 was three and the pitch p of the ribs 3, 4 on each side was 3.56 mm.Further, the height hr (hr1 or hr2) of each rib 3, 4 was 2.5 mm, theangle of each rib 3, 4 was 40 degrees, the belt length was 900 mm andthe belt thickness H was 4.8 mm.

Then, the above V-ribbed belt sample of the present invention was woundbetween a pair of 120 mm diameter V-ribbed pulleys arranged vertically,and was put into “a reverse bent state” with the slack-side back facethereof pushed by a 60 mm diameter tension pulley. In this state, thebelt was left to stand in a low-temperature atmosphere of −35° C. for 24hours with a load of 298N applied to the lower V-ribbed pulleydownwardly (in a direction to increase the belt tension). Next, bysetting one cycle to a period of time during which the upper pulley isdriven into rotation at 2000 rpm for 5 minutes and is then held into astop for 25 minutes, the number of cycles taken for the belt to producea crack in the rib was measured.

This examination was repeated under the above conditions while changingthe ratio of the curvature radius R of the corner at the rib top to therib height hr, thereby obtaining results shown in FIG. 9. Further, therates of slip of the belt in these conditions are also shown in a brokenline in FIG. 9. As can be seen from FIG. 9, when the ratio of thecurvature radius R of the corner to the rib height hr satisfies 0.17hr≦R<0.5 hr, the number of cycles taken for the belt to produce a crackis maximum, i.e., the time lapsed for the occurrence of a crack isextended. At the same time, the rate of slip is 2% or less whichapproximates a reference value. Accordingly, a belt slip is difficult tooccur and improved power transmission performance of the belt can beensured.

Furthermore, the above examination was repeated under the sameconditions while changing the ratio of the curvature radius R of thecorner at the rib top to the rib pitch p, thereby obtaining resultsshown in FIG. 10. It can be found by the results that when the ratio ofthe curvature radius R of the corner to the rib pitch p satisfies 0.14p≦R<0.35 p, the number of cycles taken for the belt to produce a crackis maximum, i.e., the time lapsed for the occurrence of a crack isextended, which improves crack-proof performance.

As can be understood from the above, when 0.17 hr≦R<0.5 hr or 0.14p≦R<0.35 p is satisfied, improvement in crack-proof performance of therib in “the reverse bent state” of the belt and improvement in powertransmission performance of the belt resulting from ensuring a largeworking flank of the rib can be achieved together.

What is claimed is:
 1. A V-ribbed belt in which a cord is embeddedgenerally in a helical arrangement to form a row along the belt widthand a plurality of ribs of approximately trapezoidal section are formedon each of bottom and back faces of the belt and in correspondingpositional relation between the bottom and back faces to extend inparallel with each other along the belt length, wherein the ribs in boththe bottom and back faces are made of rubber of identical characteristicand formed in approximately identical size and shape, and the cord isdisposed substantially in the middle of the belt thickness, wherein atop of the rib comprises a planar top surface along the belt width and apair of right and left corners with one end continuing to a left orright end of the top surface and the other end continuing to a side faceof the rib, both the corners are formed in respective arcuate sectionshaving different centers of curvature positioned bilaterally withrespect to the centerline of the rib, and when the height of the rib ishr and the curvature radius of the corner of the rib is R, the curvatureradius R is set to meet the following formula: 0.17 hr≦R<0.5 hr.
 2. TheV-ribbed belt of claim 1, wherein a difference between a belt lengthwhen the ribs in the bottom face of the belt are faced on an innerperipheral side thereof and a belt length when the ribs in the back facethereof are faced on the inner peripheral side thereof is 2 mm or less.3. The V-ribbed belt of claim 1, wherein a difference between a distancefrom the center of the cord to a top end of each of the ribs in thebottom face and a distance from the center of the cord to a top end ofeach of the corresponding ribs in the back face is 0.3 mm or less on theaverage over the entire circumference of the belt.
 4. A V-ribbed belt inwhich a cord is embedded generally in a helical arrangement to form arow along the belt width and a plurality of ribs of approximatelytrapezoidal section are formed on each of bottom and back faces of thebelt and in corresponding positional relation between the bottom andback faces to extend in parallel with each other along the belt length,wherein the ribs in both the bottom and back faces are made of rubber ofidentical characteristic and formed in approximately identical size andshape, and the cord is disposed substantially in the middle of the beltthickness, wherein a top of the rib is formed of a planar top surfacealong the belt width and a pair of right and left corners with one endcontinuing to a left or right end of the top surface and the other endcontinuing to a side face of the rib, both the corners are formed inrespective arcuate sections having different centers of curvaturepositioned bilaterally with respect to the centerline of the rib, andwhen the pitch of the ribs is p and the curvature radius of the cornerof the rib is R, the curvature radius R is set to meet the followingformula: 0.14 p≦R<0.35 p.
 5. The V-ribbed belt of claim 4, wherein adifference between a belt length when the ribs in the bottom face of thebelt are faced on an inner peripheral side thereof and a belt lengthwhen the ribs in the back face thereof are faced on the inner peripheralside thereof is 2 mm or less.
 6. The V-ribbed belt of claim 4, wherein adifference between a distance from the center of the cord to a top endof each of the ribs in the bottom face and a distance from the center ofthe cord to a top end of each of the corresponding ribs in the back faceis 0.3 mm or less on the average over the entire circumference of thebelt.